Selective deposition of metal

ABSTRACT

Method of selectively electrolessly depositing a metal of the type that can be deposited on a catalyst-treated substrate, on exposed substrate areas, only, of an insulating substrate which also has areas covered with a synthetic resin-type photoresist, comprising successively treating the entire surface with a sensitizing solution and with an activating solution each of which contains a small amount of an octyl phenol ethylene ethanol containing five to 13 ethylene oxide groups, and then treating the entire surface with a solution for electrolessly depositing the metal, whereby the metal deposits only on the exposed substrate areas and not on the photoresist.

United States Patent DiStefano et al. Feb. 8, 1972 [54-] SELECTIVE DEPOSITION OF METAL 3,075,855 H1963 Agens.... ..1 17/212 X [72] Inventors: Ralph David DiStefano, Mercerville; Ed- 0TH ER PU BLICATIONS ward Anthony James, Skillman, both of Coucoulas et al., Selective Electroless Plating" Technical 73 Assignecz RCA Corporation Digest N0. 9 Jan. i968 pp. 35-36 22 Filed: Aug 6 19 9 Primary Examiner-Ralph S. Kendall Assistant Examiner-Alan Grirnaldi PP 843,088 Attorney-Glenn H. Bruestle [57] ABSTRACT [52] US. Cl. ..ll7/212,96/36.2, ll7/47A 51] Method of selectively electrolessly depositing a metal of the [58] Field of Search ..1 17/212, 47 A,-l60; 96/36.: type that can be deposited on a catalyst-treated substrate, on exposed substrate areas, only, of an insulating substrate which [56] References Cited also has areas covered with a synthetic resin-type photoresist,

comprising successively treating the entire surface with a sen- UNITED STATES PATENTS sitizing solution and with an activating solution each of which contains a small amount of an octyl phenol ethylene ethanol 3 328 a G containing five to l3 ethylene oxide groups, and then treating 0 er ct d 17/160 X the entire surface with a solution for electrolessly depositing 5/969 MFCOmaC "117/212 the metal, whereby the metal deposits only on the exposed 3,376,139 4/ I968 I Giangualano et al.. ..96/362 substram areas and not on the photoresist 3,269,861 8/1966 Schneble ..1 [7/213 3,119,709 H1964 Atkinson l 17/47 4Claims,No Drawings SELECTIVE DEPOSITION OF METAL BACKGROUND OF THE INVENTION Patterns of electrical conductors on insulating substrates have been made by a number of different methods. One of these is to cover the substrate with a metal foil, then deposit a pattern of an etch-resist on all areas where a metal conductor is desired, and finally etch away the metal from all areas not covered with the resist. This method is adequate only for making printedcircuits which do not have high resolution requirements. Moreover, a large amount of metal is either wastedor requires additional expense to recover it from the spent etching solution.

Other previously known methods utilize electrolytic or electroless deposition=of metal instead of laminated metal foil. In one electroless deposition method, a pattern of resist is laid down on an insulating substrate. The entiresurface, including both resist and exposed substrate areas, is treated with a sensitizer and an activator, and then the entire surface is treated with the metal depositing solution. Metal deposits both on the substrate and on the resist-covered areas and the resist withits metal coatingis later removed to leave only the metal pattern on the substrate. In this method, the edges of the conductors have a tendency to be ragged and hence resolution is not as good as required for some uses.

Another electroless deposition method involves first sensitizing and activating the entire substrate surface before putting down a-resist pattern. Then after putting downthe resist pattern, the entire surface is treated with theelectroless metal-depositing solution. In this method, metal deposits only on the sensitized and activated exposed substrate areas and not on the resist-covered areas. Resolution is excellent since the edges of the resist act as forms for the plating. But this method, also, has a serious disadvantage in that sensitization and activation-of the substrate surface involves deposition of metal ions, usually palladium, thereon. This produces paths of relatively low resistivity between the deposited conductors and, if conductor spacing is very close, may cause electrical breakdown.

DESCRIPTIONOF PREFERRED EMBODIMENT EXAMPLE I The invention will first be described in connection with deposition of a defined pattern of copper conductors ona ceramic plate. A beryllia ceramic plate is first scrub cleaned with an abrasive cleaner such as Ajax cleanser.

The cleanedsubstrate is then coated withapolyvinyl cinnamate type photoresist, such as KPR (Eastman Kodak Co.).

After drying, the coated'substrate is given a first bake in-air at a temperature of about 65 C. for 2 hours. The substrate is then-recoated with the photoresist and givena second bake'at about 65 C. for 2 hours, followed by a 20 minute bake at about90 C.

Next, the conducting pattern is defined by exposure to a carbon'arc of other UV source through a master. Where the light strikes, the resist is hardened. The resist is then developed with a conventional developer to lay bare'thesub strate by removal of resist that was not exposed tolight.

The substrate is then cleansed in an aqueous solution comprising l'g. trisodium phosphate and I0 g. sodium hydroxide per liter. The treatment is preferably carried out at 5560 C. for at least 8 minutes. After this cleaning treatment, the substrate is thoroughly rinsed in water.

The substrate is next subjected to a two-step etching treatment to remove surface contaminants. The firstof these steps is treatment with a 20-30'percent by weight aqueous solution of ammonium bifluoride at room temperature for about minutes. After thorough rinsing in water, the substrate is treated for 3 minutes with an aqueous solution containing.8 percent by weight concentrated hydrofluoric acid and 20 'percent by weight of ammonium-fluoride.

After the etching treatment, the substrate is rinsed ultrasonically for l minute in water.

Next, the substrate is immersed in -a sensitizing solution which has been modified. so that the ceramic substrate is sensitized. but not the resist. The sensitizing solution comprises 10 g. SnCl '2H O, 10 ml. concentrated. HCl and 2 drops of octyl phenol ethylene ethanol containing 12 or 13 ethylene oxide groups (TritonX-lOZ). I

Then the substrate is immersed in an activating solution which activatesthe ceramic substrate'and not the resist. The activator solutioncomprises 1 percent by weight PdClfll-l O andicontains 2' drops/100 ml. of octyl phenol ethylene ethanol containing l2 or 13 ethylene oxide groups. The latter substance in both the sensitizing solution and the activator solutionlimits the action of the sensitizing and activating material to the substrate in a manner notfully understood.

After rinsing the substrate once more ultrasonically in water, it is now immersed in an electroless plating bath.

lnthis example, the bathcomprises:

Cu(NO,),6H,) l5 g./|. NaHco, l0 g./l. Rochelle salt 30 .g./l. NaOH 20 g./l. Formaldehyde (37 100 ml./l. pH ll.5

Temperature 25 C.

Copperis deposited on. the exposed substrate at a rate of about--.0.l mil/hr. A- thickness of about 0.5 mil. is deposited. Copper doesnotdeposit onthe photoresist surfaces.

Thesubstratemay now be stripped. of photoresist and used, or the copper may be built. up thicker by electroplating, or another metalmay be electroplated over the copper.

EXAMPLE II The method of the invention can also be usedv to deposit a pattern of metal conductors'on a substrate which comprises fiberglass in anepoxy resin-binder.

Prior to the ,depositionof the metal, the substrate is cleaned, coated withKPR .photoresist, baked, exposed to a pattern of light. and shadow, developed and again cleaned and then etched, all asin Example I. The board is then sensitized and activated withv solutions containing2 drops/l00'mil of octyl phenolethylene ethanol containing five ethylene oxide groups (Triton-.N-S7).

Metal is then deposited electrolessly using any one of a number of conventionalbaths. A typical bath comprises:

Nisommo as g. Sodium citrate 10 g. Sodium acetate 10 g. Sodium'hypcphosphite 15 g. MgSO, 20 g.

Water 1,000 ml.

This bath is preferably used at 8590 C. Othernickelbaths canalso be used. Another preferred bath is onewhich is usable at room temperature. This may comprise:

26 g. ll. of bath NalhPOdIgO 25 g. ll. of bath NmlhOrlOI-I 50 g. ll. of bath Ammonium hydroxide containingSS by wt. when 20 cc.ll. of bath in addition to copper and nickel, other metals, such as cobalt; which can be deposited electrolessly on a substrate which has been treated witha deposition catalyst (activator) can also be selectively'deposited by the method of the invention'.

Besides stannous chloride, other conventional sensitizers, usually othertin compounds, can be used in the present methotLAlso, other activators such as platinum chloride or gold chloridecanbeused.

Theblockingtactionof the wetting agent'on the photoresist does not appear to'bev affected'by the particular composition of the'sensitizer, the activator or the metal being deposited.

Any conventional photoresist which is basically a synthetic resinous material or a synthetic resinlike material can be util-,

Although the examples of practicing the invention have been directed to depositing a pattern of conductors on an insulating substrate, the method has also been found to be especially advantageous for depositing metal inside holes in circuit boards which are intended to connect patterns on opposite sides of the substrate or connect different layers of multilevel systems. Prior to the present invention, it was conventional to sensitize and activate an entire board including the holes. Then a resist was silk screened on the board where metal was not to be plated. One of the problems was to deposit the resist without blocking any of the holes. But this has proved difficult to do because of the problem in registering the screen with the necessary degree of precision. In the present process a photoresist is used and master patterns can be registered with a very high degree of accuracy.

We claim:

1. A method of selectively electrolessly depositing a metal on exposed areas only of an insulating substrate selected from the class consisting of ceramics and fiberglass-filled. epoxy resins having areas adjacent to said exposed areas covered with a photoresist of the type comprising a synthetic resinous material selected from the class consisting of polyvinyl cinnamate and polyolefin material, said method comprising:

a. treating both said exposed areas and said resist-covered areas with a solution of a sensitizer containing an effective amount of the order of 2 drops per ml. of solution of octyl phenol ethylene ethanol containing five to 13 ethylene oxide groups,

b. then treating both said exposed areas and said resistcovered areas with a solution of a deposition activator containing an effective amount of the order of 2 drops per 100. ml. of solution of octyl phenol ethylene ethanol con taining five to 13 ethylene oxide groups, and then c. plating metal electrolessly on said exposed areas whereby said metal does not deposit on said resist-covered areas.

2. A method according to claim 1 in which said metal is nickel and said insulating substrate is a ceramic.

3. A method according to claim 2 in which said octyl phenol ethylene ethanol contains 12 or 13 ethylene oxide groups.

3. A method according to claim 2 in which said sensitizer is stannous chloride and said activator is palladium chloride.

Patent No Col. 1,

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(SEAL) Attest:

Attesting Inventor(s) UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION line 57 change line 20 change line 58 change line 60 change EDWARD M.FLETCHER,JR.

Officer Dated Ralph David DiStefano & Edward Anthony James It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

"of" to or "Cu(NO .6H 130 Cu (NO .6H O

"NiSO .H

Signed and sealed this 20th day of June 1972.

ROBERT GOTI'SCHALK Commissioner of Patents FORM I O-1050 (10-69) USCOMM-DC 50375-P69 U.5. GOVERNMENT PRINTING OFFICE 1959 0-366-335 

2. A method according to claim 1 in which said metal is nickel and said insulating substrate is a ceramic.
 3. A method according to claim 2 in which said octyl phenol ethylene ethanol contains 12 or 13 ethylene oxide groups.
 4. A method according to claim 2 in which said sensitizer is stannous chloride and said activator is palladium chloride. 